Support Etcd as Service Registry

[l-duan]

Why do you need it?

etcd is a distributed key value store that provides a reliable way to store data across a cluster of machines. etcd gracefully handles leader elections during network partitions and will tolerate machine failure, including the leader.

How could it be?

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[Charlie17Li]

@l-duan 请问什么场景会使用 etcd 作为注册中心，据我所知，etcd 自身只是存 kv，通常需要依赖其他工具做注册中心，例如dubbo；但这就会引入一个问题，每种工具使用 etcd 的姿势不太一样，例如存储的"Service"内容大相径庭；如果支持etcd的话，是不是得统一下 “Service” 规范